1. Field of the Invention
The present invention relates to a magnetic sensor, a magnetic field sensing method, a magnetic recording head, and a magnetic memory device such as a magnetic random access memory.
2. Description of the Related Art
Since the advent of a GMR head utilizing the giant magnetoresistive effect (GMR effect) as a magnetic read head, the recording density of magnetic recording has increased at an annual rate of about 100%. The GMR head includes a composite film, i.e., a magnetic multilayer, of a sandwich structure of ferromagnetic layer/nonmagnetic layer/ferromagnetic layer. With the GMR head, one ferromagnetic layer is subjected to an exchange bias to pin its magnetization, and the other ferromagnetic layer is subjected to an external magnetic field to change the direction of its magnetization. A change in the relative angle between the magnetization directions of the two ferromagnetic layers is detected as a change in resistance. So far, a CIP (Current-In-Plane)-GMR element and a CPP (Current-Perpendicular-to-Plane)-GMR element have been developed. With the CIP-GMR element, a current is caused to flow in the magnetic multilayer plane to detect a change in resistance. With the CPP-GMR element, a current is caused to flow perpendicularly to the magnetic multilayer plane to detect a change in resistance. These GMR elements exhibit a magnetoresistance effect of about 10% and are expected to allow for a recording density of up to about 200 Gbit/inch2 (Gbpsi).
To allow for magnetic recording at higher recording densities, a TMR element has been under development which utilizes the tunneling magnetoresistance (TMR) effect. The TMR element includes a composite film of ferromagnetic layer/insulator layer/ferromagnetic layer. A voltage is applied between the ferromagnetic layers to cause a tunnel current to flow. The TMR element utilizes the fact that the magnitude of the tunnel current changes with the magnetization directions of the upper and lower ferromagnetic layers and detects a change in the relative angle between the magnetization directions of the ferromagnetic layers as a change in tunnel resistance. The TMR element, being larger in MR ratio than the GMR element (about 50% at maximum) and high in signal voltage, is expected to allow for a recording density of about 300 Gbpsi.
With magnetic recording at more than some hundreds of Gbpsi, the bit size ranges from tens of nm to 100 nm. To avoid thermal fluctuation of magnetization, therefore, it is required to use a magnetic material which is large in coercivity for the magnetic recording layer. It has been proposed to perform thermally assisted recording on such a medium. With thermally assisted recording, the medium is heated to lower its coercivity and then subjected to a recording magnetic field. More specifically, to realize high-speed and localized recording, laser-based thermally assisted recording has been proposed which irradiates a medium with a laser beam having a large power density to heat it (T. Rausch, Jpn. J. Appln. Phys., 42 (2003) pp. 989-994). With this laser-based thermally assisted recording, however, it is difficult to control successfully heating of the medium and application of a magnetic field to the medium.
In addition, the TMR element has a problem that a shot noise component in an output signal is large and hence the S/N ratio (signal-to-noise ratio) cannot be improved. The shot noise is attributed to current fluctuation produced by electrons passing through the tunnel barrier irregularly. In order to suppress the shot noise and obtain a desired signal voltage, it is required to reduce the thickness of the tunnel insulating layer and thereby lower the tunnel resistance. When the thickness of the tunnel insulating layer is reduced, however, short-circuiting of the upper and lower ferromagnetic layers is liable to occur, lowering the magnetoresistance ratio (MR ratio). For this reason, it is difficult to fabricate a TMR element which exhibits good characteristics even at high recording densities.
Furthermore, with a magnetic random access memory (MRAM) in which the recorded magnetization of the ferromagnetic layers of each TMR element is used as recorded data, it is pointed out that, when its packing density is increased, the current magnetic field for writing increases.
In recent years, magnetic white noise has become a problem in common with the GMR and TMR elements. Unlike electrical noise such as the aforementioned shot noise, the white noise is caused by thermal fluctuation of micro-magnetization. For this reason, the white noise becomes more dominant as the elements become smaller in size. With elements adapted for 200-300 Gbpsi, therefore, it is supposed that the magnetic white noise will be greater in influence than the electrical noise. For example, a study is known by which high-frequency noise of a magnetic multilayer element is measured as a function of applied magnetic field and the magnetic resonance frequency of the ferromagnetic layer is examined (N. Stutzke et al., Applied Physics Letters, vol. 82, No. 1, (Jan. 6, 2003) pp. 91-93).